Fault tolerant electrical circuit and method

ABSTRACT

An electrical circuit and method substantially to mitigate the effects of a current increase due to a fault within the circuit. In particular, where the electrical circuit ( 80 ) includes a laser diode it is desirable to create a fault tolerant circuit to avoid a sudden increase in light intensity output by the laser diode. A track ( 44   b ) associated with the laser diode is identified and insulated by means of a layout of the circuit. Specifically, where the circuit is an integrated circuit, metal layers ( 42, 44, 46 ) and vias ( 50 ) are utilised to form an insulating shield ( 76 ) around the track ( 44   b ) associated with the laser diode.

The present invention relates, in general, to an electrical circuit andmethod for mitigating the effects of a current increase due to a faultwithin the circuit. The invention is particularly, but not exclusively,concerned with a current increase across a laser diode due to anelectrical short within the circuit.

In the field of integrated circuits, it is known that faults may occurin the structure of an integrated circuit. These structural faults mayoccur during the manufacturing process or alternatively they may ariseduring use either as a result of material weaknesses or misuse of thecircuit. It is very difficult to locate all faults in an integratedcircuit prior to use. Unfortunately, undetected faults may cause theintegrated circuit (and any electrical devices that it is coupled to) tofail in their operation.

It is known in the art to design electrical circuits which are ‘faulttolerant’. A fault tolerant circuit is generally configured so that afailure of strategic components does not result in the complete loss ofcircuit operation.

Where an integrated circuit includes a laser diode within the circuitry,undetected faults may be especially problematic. For example, if theintegrated circuit were shorted to ground as a result of a component ofthe integrated circuit failing, then a significant increase in currentacross the laser diode may occur. The current increase may cause lightintensity emitted from the laser diode to increase because the lightintensity output from the laser diode is proportional to the currentdrawn. An increase in light intensity output may represent significantdanger to a user of the laser diode circuit, the danger arising frominadvertent projection of the laser light into the user's eye. In thisregard, all laser circuits must be stringently tested for compliancewith stipulated regulatory requirements, which testing is arduous andtherefore expensive. In any event, even if a circuit passes theregulating (safety) tests, there is no guarantee that a fault will notemerge with time with a particular device, which fault could generate alocalised current that drives the laser divide output beyond stipulatedlight intensity outputs and into an unsafe operational state.

There is therefore a need to produce an electrical circuit whichmitigates the problem of an increase in current resulting from anundetected or unexpected electrical fault.

According to a first aspect of the present invention, there is providedan electrical circuit containing a first circuit having associatedtherewith a first track supporting, in use, a first current; and asecond circuit drawing, in use, a second current, the second circuitlocated proximate to the first track, the electrical circuitcharacterised by an electrical shield providing an electrically isolatedenclosure, the electrical shield positioned substantially about thefirst track and such that the shield inhibits, in use, shorting of thefirst track to the second circuit to restrict, in use, substantialsumming of the first current with the second current.

The shield, in use and under fault conditions, may inhibit establishmentof a short circuit supporting flow of a current greater than apredetermined threshold through an electrical component.

The electrical component may be a laser diode, and the second circuitmay be a track. Also, the shield may comprise at least one metal layerwithin an integrated circuit or printed circuit board, the shieldfurther including at least one via.

In a second aspect of the present invention there is provided anelectrical device comprising the electrical circuit of the first aspectof the present invention.

In a third aspect of the present invention there is provided anintegrated circuit or printed circuit board comprising the electricalcircuit of the first aspect of the present invention or the electricaldevice of the second aspect of the present invention.

In a fourth aspect of the present invention there is provided a methodof mitigating effects of a short circuit fault condition within anelectrical circuit, the method comprising determining a currentsensitive circuit; and providing a ground insulated shield substantiallyabout said current sensitive circuit to prevent, in use, a short circuitfault condition associated with a second electrical circuit fromincreasing current through the current sensitive circuit.

The invention may also comprise a method of laying out the electricalcircuit such that at least one metal layer and at least one via of thenon-critical track form a shield around a greater part of the determinedcritical track.

Advantageously, a fault tolerant integrated circuit may be achieved bythe layout of the circuit components and, where the circuit includes alaser diode, a sudden increase in light intensity output by the laserdiode (which may damage the eyes of the user) may be avoided.Specifically, a track including the laser diode is identified andinsulated by means of a layout of the circuit (metal layers and vias areutilised to form an insulating shield around the laser diode).

The present invention is generally applicable to electrical circuitswhich require protection from current overload and, whilst beingparticularly applicable to integrated circuits which include at leastone laser diode, can be employed more widely.

An embodiment of the present invention will now be described by way ofexample only and with reference to the accompanying drawings, in which:

FIG. 1 is a schematic diagram of a layout of an integrated circuit ofthe prior art;

FIG. 2 is a schematic diagram of a front view of a verticalcross-section through an embodiment of the present invention;

FIG. 3 is a schematic diagram of a layout of an integrated circuit of analternative embodiment of the present invention;

FIG. 4 is a schematic diagram of a side view of a vertical cross-sectionthrough the 3-metal system of FIG. 2;

FIG. 5 is a schematic diagram of a circuit layout incorporating aninsulated track system of the present invention; and

FIG. 6 exemplifies a typical circuit configuration in which the presentinvention is implemented.

FIG. 1 illustrates a vertical cross-sectional view through a knownintegrated circuit 10. Metal layer 12 and metal layer 14 (each metallayer including at least one track, i.e. a conducting path) areseparated by a first layer of insulation (e.g. oxide) 18 within anintegrated circuit of the like. Metal layer 14 and metal layer 16 areseparated by a second insulation layer 20. In this three-metal system,metal layer 12 is electrical ground. A via (which is an electricalconnection between layers of metal) 22, 24 connects layers 12 and 14,and layers 14 and 16 respectively. A track on metal layer 14 isconnected to a laser diode (not illustrated) that is typically externalto an integrated circuit (IC).

In operation, a fault within the integrated circuit 10 may result in anunwanted electrical bridging between two of the metal layers 12, 14, 16as the circuit shorts to ground. For example, if a fault occurred withinmetal layer 14, this may result in a significant increase in currentacross components (including the laser diode) in the track in metallayer 14. The resulting increased light intensity may be damaging to auser's eyes or have other unwanted detrimental effects depending on theuse of the integrated circuit.

FIG. 2 illustrates a front view of a vertical cross-section through anintegrated circuit 40 incorporating the concepts of the presentinvention. Metal layer 42 is grounded and metal layer 44 supportsseparate tracks 44 a, 44 b and 44 c. The track contained in metal layer44 b is identified as being strategically significant to the functioningof the integrated circuit 40, since track 44 b draws an amount ofcurrent that could combine with secondary current in another independentcircuit in a short circuit environment, to present an increased currentsource to and through a laser diode. A metal layer 46 and the metallayer 42 are connected to tracks 44 a and 44 c with vias 50. Thus, ametal shield is created around the strategically current-significanttrack 44 b, thereby electrically insulating or isolating the currentfrom shorting through the laser diode.

In operation, a fault within the integrated circuit 40 may result in anunwanted electrical bridging between two of the metal layers 42, 44, 46as the circuit shorts to ground. However, the operationally sensitivetrack 44 b will not be affected by the short because it is effectivelyinsulated by the metal shield created by the layout of the integratedcircuit.

FIG. 3 illustrates a vertical cross-sectional view of an integratedcircuit of an alternative embodiment of the present invention. Metallayer 62 is grounded and metal layer 64 contains separate tracks 64 a,64 b and 64 c. The track contained in metal layer 64 b is identified asbeing strategically significant to the functioning of the integratedcircuit 60, since track 64 b draws an amount of current that couldcombine with secondary current in another independent circuit in a shortcircuit environment, to present an increased current source to andthrough a laser diode. A plurality of vias 68 extend from tracks 64 aand 64 c. One of said vias 68 is connected between track 64 a and themetal layer 62 and another one of said vias 68 is connected betweentrack 64 c and the metal layer 62. Thus, a metal shield is createdaround the strategically current-significant track 64 b, therebyelectrically insulating or isolating the current from shorting throughthe laser diode.

Thus, the sensitive track 64 b is insulated on three sides by a groundedmetal shield, leaving the upper side uninsulated. In operation, bridgingto the track 64 b is avoided as it is only necessary to have insulationbetween tracks of a single layer but not at both the upper and lowersides. Therefore, a further embodiment of the present inventioncomprises the sensitive track insulated on three sides by a groundedmetal shield, leaving the lower side uninsulated.

FIG. 4 illustrates a side view of a vertical cross-sectional viewthrough a multilayer integrated circuit 70 incorporating the concepts ofthe present invention. A first circuit 72, in use, generally drawsrelatively high levels of current, but at least significant amounts ofcurrent that warrant concern in relation to laser light intensity outputif the first circuit electrically shorts, i.e. combines with a secondcircuit 74. One of the first or second circuits will have a conductionpath to a laser diode that affects, i.e. provides, operating current tothe laser diode. An electrically insulated shield 76 realised by metaltracks or layers 44 a and 44 c therefore isolates the first circuit 72from the second circuit under fault or multi-fault conditions to ensurethat current through the laser diode never exceeds predetermined safetylevels. In other words, the shield 76 protects against a short between“Signal A” and “Signal B”. The shield 76 may include vias (not shown forthe sake of clarity). Operation of a circuit including this componentlayout is described above with reference to FIG. 2. The shield 76 istied to a safe (i.e. stable) potential, typically selected to be groundpotential.

FIG. 5 illustrates a plan view of an integrated circuit (IC) 80incorporating a track 44 b insulated according to a preferred embodimentof the present invention. Bond pads 92 are typically spaced along aperiphery of the IC 80. To exemplify the requirements for deployment ofthe present invention, it is useful to consider the circuit scenariowhere a bond pad 93 in the top left-hand corner of the IC 80 isconnected to first circuit 72 via insulated track 44 b. The track 44 bis insulated by a shield 76 (as illustrated in FIG. 2 or FIG. 3) as thetrack 44 b traverses a second circuit 74. The second circuit 74 receivesdrive current from a current supply 88 which, in turn, is coupled to thefirst circuit 72. A laser diode 90 is coupled between the first circuit72 and the second circuit 74 using bond pads to provide an off-chipconnection. Clearly, the IC 80 also includes further trucks that connectbond pads to the components mounted on the IC 80.

In operation, bridging between the second circuit 74 and the sensitivetrack 44 b is avoided due to the insulating shield 76. Therefore, if afault occurs between the first circuit 72 and the second circuit 74, theshield 76 acts to prevent any significant (and preferably any absolute)increase in light intensity output by the laser diode 90 (as aconsequence of increased current through the laser diode arising byvirtue of a short-circuit. The shield 76 preferably entirely enclosesthe track 44 b, although the degree of encapsulation is a design featuredictated by the sensitivity of the track 44 b in terms of its currentshorting capabilities. In other words, the shield 76 may be sufficientif it substantially but not totally encapsulates the track 44 b, withthe shield acting to provide the requisite electrical isolation byvirtue of its physical location and presence.

Looking briefly at FIG. 6, the present invention is shown implementedwithin a typical circuit 100. For the sake of explanation, the firstcircuit is coupled to trace A (reference numeral 102), the traceproviding current to a first current mirror circuit 104 coupled toground 106 via resistive networks. The second circuit 74 is coupled totrace B (reference numeral 108), the trace connected to a second currentmirror 110 coupled to a power supply 112 through an appropriateresistive network. The first circuit 72 and the second circuit arecoupled together through laser diode 90. In absence of the shield 76 ofthe present invention, any short between trace A and trace B couldresult in a near infinite amount of current flowing through the laserdiode 90.

In summary, according to an underlying inventive concept, a system of apreferred embodiment functions to mitigate the effects of a currentincrease due to a fault within an integrated circuit.

It will be appreciated that the above description has been given by wayof example only and that modifications in detail may be made within thescope of the invention. For example, whilst the present invention hasbeen generally described in relation to a three-metal system of anintegrated circuit, the underlying concept can be employed in integratedcircuits comprising a different number of metal layers (e.g. a six-layerdesign). Also, the integrated circuit of the present invention may havea layout wherein any part of the metal shield is connected to ground.Furthermore, while the present invention has particular applicability tolaser diode circuits, it will be appreciated that the inventive conceptof shielding one track from another to avoid an excess current conditionin a circuit is more generally applicable (even to the extent that theshielding prevents damage and protects a costly (expensive) discretedevice or the like). Indeed, the present invention can find applicationin printed circuit boards (PCBs).

What is claimed is:
 1. An electrical circuit comprising: a first circuithaving associated therewith a first track for supporting, in use, afirst current; and a second circuit for drawing, in use, a secondcurrent, the second circuit located proximate to the first track; anelectrical shield providing an electrically isolated enclosure, theelectrical shield entirely enclosing the first track, the shield beingarranged for preventing, in use, short circuiting of the first track tothe second circuit for restricting, in use, substantial summing of thefirst current with the second current.
 2. The electrical circuit ofclaim 1, wherein the shield, in use and under fault conditions, isarranged for inhibiting establishment of a short circuit supporting flowof a current greater than a predetermined threshold through anelectrical component.
 3. The electrical circuit of claim 2, wherein theelectrical component includes a laser diode.
 4. The electrical circuitof claim 1, wherein the second circuit includes a track.
 5. Theelectrical circuit of claim 1, wherein the shield comprises at least onemetal layer within an integrated circuit and at least one via.
 6. Anelectrical device comprising the electrical circuit of claim
 1. 7. Anintegrated circuit or printed circuit board comprising the electricaldevice of claim
 6. 8. An integrated circuit or printed circuit boardcomprising the electrical circuit of claim
 1. 9. An integrated circuitcomprising a substrate, peripheral bonding pads on the substrate, alaser diode having first and second electrodes respectively connected tofirst and second of the bonding pads, a first circuit mounted on thesubstrate, the first circuit having (a) a first terminal connected to agrounded bonding pad and (b) a second terminal connected to the secondbonding pad and thence to the first electrode of the laser diode forsupplying current to the laser diode, a second circuit mounted on thesubstrate having a terminal connected to one of the bonding pads, aconducting track having first and second opposite ends respectivelyconnected to one of the bonding pads and a further terminal of the firstcircuit, the track including a portion extending across the secondcircuit, and a shield entirely enclosing the portion of the trackextending across the second circuit for preventing short circuiting ofthe tack to the second circuit and thereby preventing excessive currentflow through (a) the first terminal, (b) the first bonding pad and (c)the laser diode electrodes.
 10. The integrated circuit of claim 9wherein the second circuit includes at least one transistor, a trace forsupplying current to at least one transistor of the second circuit, theat least one transistor of the second circuit being connected forsupplying current to the second electrode, the track being connected tosupply current to at least one transistor of the first circuit, the atleast one transistor of the first circuit being connected for supplyingcurrent to the first electrode via, the flat bonding pad, the track andtrace being located such that a short circuit between them is likely toresult in excessive current being supplied to the laser diode via thefirst and second bonding pads, the shield being interposed between thetruck and trace for preventing such a short circuit.
 11. An integratedcircuit comprising a substrate carrying peripheral bonding pads, a laserdiode having first and second electrodes respectively connected to firstand second of the bonding pads, a first circuit mounted on thesubstrate, the first circuit having a (a) first terminal connected to agrounded bonding pad and a second terminal connected to the firstbonding pad and thence to the first electrode of the laser diode, and(b) a second terminal connected to the first bonding pad and thence tothe first electrode of the laser diode for supplying current to thelaser diode, a second circuit mounted on the substrate, the secondcircuit having a terminal connected to one of the bonding pads, aconducting track having that and second opposite ends respectivelyconnected to one of the bonding pads and a further terminal of the firstcircuit, the track including a portion extending across the secondcircuit, a shield interposed between the portion of the track extendingacross the second circuit for preventing short circuiting of the trackto the second circuit and thereby preventing excessive current flowthrough (a) the first terminal, (b) the first bonding pad and (c) thelaser diode electrodes, the second circuit including a trace forsupplying current to at least one transistor of, the second circuit, theat least one transistor of the second circuit being connected forsupplying current to the second electrode, the track being connected tosupply current to at least one transistor of the first circuit, the atleast one transistor of the first circuit being connected for supplyingcurrent to the first electrode, the trace and track being located suchthat a short circuit between them is likely to result in excessivecurrent being supplied to the laser diode via the bonding pads, thetrack being connected to supply current to at least one transistor ofthe first circuit, the at least one transistor of the first circuitbeing connected for supplying current to the first electrode via thefirst bonding pad, the shield being interposed between the track andtrace for preventing such a short circuit.